Silicon carbide, a crystalline compound of silicon and carbon, has long been known for its hardness, its strength and its excellent resistance to oxidation and corrosion. Silicon carbide has a low coefficient of expansion, good heat transfer properties and maintains high strength at elevated temperatures. In recent years, the art of producing high density silicon carbide bodies from silicon carbide powders has been developed. Methods include reaction bonding, chemical vapor deposition, hot pressing and pressureless sintering (initially forming the article and subsequently sintering). Examples of these methods are described in U.S. Pat. Nos. 3,853,566; 3,852,099; 3,954,483; and 3,960,577. The high density silicon carbide bodies so produced are excellent engineering materials and find utility in fabrication of components for turbines, heat exchange units, pumps and other equipment or tools that are exposed to severe wear and/or operation under high temperature conditions.
In order to obtain high density and high strength silicon carbide ceramic materials, various additives have been utilized. For example, a method of hot pressing silicon carbide to densities in order of 98 percent of theoretical by addition of aluminum and iron as densification aids is disclosed by Alliegro, et al., J. Ceram. Soc., Vol. 39, No. 11, November, 1965, pages 386 to 389. They found that a dense silicon carbide could be produced from a powder mixture containing 1 percent by weight of aluminum. Their product had a modulus of rupture of 54,000 psi. at room temperature and 70,000 psi at 1371.degree. C.
Aluminum nitride, a crystalline compound of aluminum and nitrogen, is widely used as a refractory material. Aluminum nitride exhibits an excellent resistance to molten metals and is an especially useful refractory for molten aluminum. Aluminum nitride has good thermal shock resistance, good strength and excellent resistance to most chemicals.
A number of proposals have been made to combine silicon carbide and aluminum nitride to produce an improved high density body which may be fabricated into articles which will withstand severe operating conditions, or into refractory materials which offer the strength of silicon carbide and the inertness of aluminum nitride. Such combinations have been proposed to improve the electrical conductivity of silicon carbide resistance elements and better the resistance of silicon carbide to corrosion at elevated temperatures. Examples of such mixtures are found in U.S. Pat. Nos. 3,259,509; 3,287,478; and 3,492,153. However, the previously proposed products are mixtures of silicon carbide and aluminum nitride which do not sinter to produce a high density, co-sintered product. One negative characteristic of aluminum nitride is that it is soluble in warm water. This characteristic has lessened the use of aluminum nitride in many applications in which it would otherwise be utilized to an advantage. The mixtures proposed in the prior art have not substantially remidied the solubility of aluminum nitride in warm water. It has now been found that a co-sintered silicon carbide - aluminum nitride product, containing up to about 45 percent be weight aluminum nitride, may be produced which has the positive attributes of both silicon carbide and aluminum nitride and which is substantially insoluble in warm water.